The free Twitter #music apps for iOS and the web (no Android version yet) hooks into external music services to let you play music: previews from iTunes or full tracks from Rdio or Spotify, if you have an account with those. Twitter says it will “continue to explore and add other music service providers.” Playing music is nothing new, of course, but what is new with this app is how you find that music.

Unlike Facebook’s scrobbling-style approach, which lets it broadcast what you’re listening to to all your friends, Twitter #music only knows what people do with music within Twitter itself. That means it will work better for you if you follow the artists you like, and your friends like to post lots of songs. Not only can you see what your Twitter followees are “now playing,” but it will help Twitter #music’s suggestions section do a better job of recommending new things to listen to.

Still, even if you don’t follow artists, and you and your Twitter friends don’t post much music, you can still get something out of these apps. They prominent include charts for the most popular and fastest-rising artists being tweeted at that time. And when you listen to an artist you find that way, you have a chance to follow them (from your main Twitter account), thereby improving the experience for later. You can also see which artists the artists who you follow are following, offering the opportunity to flesh out your Twitter #music experience even further.

The neat, mosaic-like interface is clearly based on We Are Hunted, which Twitter acquired, and whose developers built Twitter #music. It’s a tiled wall of artist images, which you can click (on the web) or tap (in the iOS app) in order to hear instantly.

The web version will roll out over the next few hours, according to Twitter (update: it is now live for us, and includes the same screens as the iOS version, more or less). We’ve been testing the iOS version this morning. Here’s how it works:

Here’s what you see as the app loads on iOS.

The app defaults to showing the most popular songs being posted to Twitter.

When you play a song, it shows up in a little mini-player at the bottom of the screen — tap that to reveal extended options within the main player.

The dropdown menu at the top lets you navigate between the app’s main four screens — or you can swipe left or right to switch between them.

You can connect the app to Rdio or Spotify on your phone, if you have a premium account with one or both of those, allowing the Twitter #music app to play full tracks instead of iTunes previews.

Sure enough, with Rdio enabled, full tracks played, including Psy’s “Gentlemen,” which currently tops Twitter #music’s most popular chart.

Here are the emerging artists on Twitter right now. As with the Most Popular chart, this one makes sense even if you don’t use Twitter for music. that much.

Twitter #music provided way better suggestions than I thought it was, which I credit to my usage of This Is My Jam, because I otherwise don’t post much music to Twitter, and I don’t follow many artists.

Here’s what my Twitter followees have posted to Twitter lately using the #NowPlaying hashtag, or from within this app.

When I tap on Peter Kafka’s song, the player expands to reveal a play button, which is a nice balance of functionality and clutter-avoidance.

Aside from the main four screens (popular, emerging, suggested, and now playing), you can search for any artist to see their profile page, from which you can follow them or see which artists they follow.

Here’s an artist profile.

I can play a song from any artist that artist follows, with a couple of taps.

Overall, if you have a Twitter account and iOS, and you’re a music fan, we’re hard-pressed to think of a reason why you wouldn’t want to install this app. It runs much better from a “cold start” than we thought it would, given that unlike Facebook, it doesn’t know what you’re listening to outside of Twitter. And the more you use it, the better it will get.

Again, there is no Android version, but ABC News, where Twitter announced Twitter #music, says it’s not available “yet.” It’s unclear at this point whether Androids and other non-iOS devices will be able to use this as a mobile web app via music.twitter.com, although we should find out about that later today.

I’m not following why you’d use this? I “follow” about a dozen musical people on twitter. When I click them it just plays their tracks, but I learn the same information from reading their feeds (that’s why I follow them). Then the list of new/emerging/suggested tracks is 99% stuff I don’t care about, and it would be a waste of time to listen to 100 songs from their iffy algorithm just to find 1 discovery.